1. Field of the Invention
The present invention relates generally to improved drilling fluids and methods of drilling subterranean well bores therewith, and more particularly, to such drilling fluids and methods wherein hardenable epoxy sealants contained in the drilling fluids are deposited on the walls of the well bores.
2. Description of the Prior Art
A variety of drilling fluids have been used heretofore in drilling subterranean well bores. The most commonly used such drilling fluids are solids-containing water based viscous gels which are often weighted with particulate weighting materials such as barite. Oil based drilling fluids are also utilized and are usually inverted oil-salt water emulsions containing organophilic clays, weighting materials and other additives. When a well bore is being drilled, the drilling fluid is circulated downwardly through the drill string, through the drill bit and upwardly in the annulus between the walls of the well bore and the drill string. The drilling fluid functions to maintain hydrostatic pressure on formations penetrated by the well bore and thereby prevent blow-outs and to remove cuttings from the well bore. As the drilling fluid is circulated, a filter cake of solids from the drilling fluid forms on the walls of the well bore. The filter cake build-up is a result of initial fluid loss into the permeable formations and zones penetrated by the well bore. The filter cake and gelled or partially gelled drilling fluid mixed therewith reduce additional fluid loss as the well is drilled.
After the well bore reaches its total depth, the drilling and circulation of drilling fluid are stopped. The well is then logged and a string of pipe, e.g., casing, is run into the well bore. After the pipe is run, the well bore has heretofore been conditioned by circulating drilling fluid downwardly through the pipe and upwardly through the annulus. The conditioning has been intended to remove filter cake and gelled or partially gelled drilling fluid from the walls of the well bore. Primary cementing operations have then been performed in the well bore, i.e., the string of pipe disposed and the well bore has been cemented therein by placing a cement slurry in the annulus and allowing it to set into a hard mass therein. When the cement slurry is run down the pipe and into the annulus, the drilling fluid in the pipe and the annulus is displaced therefrom.
The purpose of cementing the above mentioned string of pipe in the well bore is to provide physical support and positioning to the pipe and seal the annulus. That is, it is intended that the set cement in the annulus will bond to the pipe and to the walls of the well bore whereby the annulus is sealed in a manner which prevents pressurized fluid migration between subterranean zones and formations penetrated by the well bore. However, the sealing of the annulus is often frustrated by filter cake and gelled drilling fluid which remain on the walls of the well bore when primary cementing operations are commenced. That is, when the cement slurry is placed in the annulus and allowed to set therein, thin layers of unconsolidated filter cake solids and gelled drilling fluid often remain between the set cement and the walls of the well bore including the faces of permeable formations or zones containing pressurized fluids. Since the inert layers of unconsolidated solids do not have the physical properties necessary to prevent pressurized fluid migration, such migration takes place by way of flow channels formed through the layers.
While a variety of techniques have heretofore been developed in attempts to remove filter cake from the walls of well bores and increase the displacement efficiencies of gelled drilling fluids therefrom prior to cementing pipe therein, continuing needs remain for improved drilling fluids, methods of drilling and methods of sealing pipe in well bores.